Hitherto, as methods of controlling the flow rate of a hydraulic cylinder which is frequently used in construction equipment, a working vehicle, industrial machinery, or the like, a method of controlling the rotation speed and rotation direction of a hydraulic pump and a method of controlling the displacement of a bi-directional dischargeable type (bent axis type, swash plate type, or the like) hydraulic pump are known.
An example of prior arts of the aforementioned method of controlling the rotation speed and rotation direction of the hydraulic pump is disclosed in International Publication WO 01/88381, and FIG. 13 shows a principal portion of a driving circuit diagram disclosed in said Bulletin. In FIG. 13, two hydraulic pumps 41 and 42 are driven by an electric motor 2. One output port of the first hydraulic pump 41 is connected to a head chamber of a hydraulic cylinder 3 via a pipe line 50, and the other output port is connected to a bottom chamber of the hydraulic cylinder 3 via a pipe line 51. One output port of the second hydraulic pump 42 is connected to the pipe line 51, and the other output port is connected to an accumulator 43 via an on-off valve 44 and a pipe line 53. The pipe line 50 is connected to the accumulator 43 via a first safety valve 45a and a first check valve 46a and through a pipe line 52. Similarly, the pipe line 51 is connected to the accumulator 43 via a second safety valve 45b and a second check valve 46b and through the pipe line 52.
According to the aforementioned constitution, when the hydraulic cylinder 3 is contracted, by rotating the hydraulic pumps 41 and 42 in a contracting direction by the electric motor 2, part of pressure oil in the bottom chamber is sucked into the first hydraulic pump 41 via the pipe line 51 and discharged from the hydraulic pump 41, and flows into the head chamber via the pipe line 50. The other excess pressure oil is sucked into the second hydraulic pump 42, discharged from the hydraulic pump 42, and accumulated in the accumulator 43 via the on-off valve 44 and the pipe line 53. On the other hand, when the hydraulic cylinder 3 is extended, by rotating the hydraulic pumps 41 and 42 in an extending direction by the electric motor 2, the first hydraulic pump 41 sucks in pressure oil in the head chamber via the pipe line 50 and discharges it to the bottom chamber via the pipe line 51. On this occasion, an equal quantity of pressure oil to compensate a deficiency in the bottom chamber is sucked from the accumulator 43 via the pipe line 53 and the on-off valve 44 and discharged by the second hydraulic pump 42, and flows into the bottom chamber via the pipe line 51. It is necessary to absorb a difference between the inflow rate and the outflow rate of pressure oil in the respective chambers which is caused by the difference in pressure receiving area between the bottom chamber and the head chamber of the hydraulic cylinder 3. Thanks to such a constitution, the difference in flow rate between the bottom chamber and the head chamber can be absorbed by two hydraulic pumps 41 and 42 and the accumulator 43.
Moreover, an example of prior arts of the method of controlling the displacement of the bidirectional dischargeable type hydraulic pump is a cylinder driving circuit shown in FIG. 14 which is disclosed in Japanese Utility Model No. 60-122579. In FIG. 14, two ports of a bidirectional dischargeable type hydraulic pump 64 are connected to a bottom chamber and a head chamber of a hydraulic cylinder 61. via a control valve 71, respectively. A rod of the hydraulic cylinder 61 is coupled to a slide of a press 62. The switching of the discharge direction of the hydraulic pump 64 is controlled by a regulator 66 to which pilot oil is supplied from a pilot pump 65. A tank 73 is coupled to the control valve 71 via a pipe line. A prefill valve 72 is provided between the bottom chamber (the slide descending side) of the hydraulic cylinder 61 and the tank 73, and when the oil feed amount from the hydraulic pump 64 is not sufficient when the press 62 is in operation, oil is supplied from the tank 73 via the prefill valve 72.
The aforementioned prior arts, however, have the following disadvantages. In a cylinder driving system disclosed in International Publication WO 01/88381, the two hydraulic pumps 41 and 42 and the accumulator 43 need to be provided in order to absorb the difference in flow rate between the head chamber and the bottom chamber of the hydraulic cylinder 3. Accordingly, the time required for piping work is greater, and moreover manufacturing costs increase. In the cylinder driving circuit disclosed in Japanese Utility Model No. 60-122579, part of oil discharged from the bottom chamber of the hydraulic cylinder 61 when the slide ascends (excess oil over the head chamber side capacity) is returned from the control valve 71 to the tank 73. Hence, there is a disadvantage that energy loss of the pressure oil in the control valve 71 is large, whereby efficient work by the hydraulic cylinder becomes impossible.